The applicant has already proposed a polytetrafluoroethylene (hereafter referred to as "PTFE") porous membrane as a filter medium used for cleaning the air in a clean room or the like (for example, JP5-202217A). The applicant has also proposed to protect the PTFE porous membrane by laminating thermoplastic, such as a spunbonded nonwoven fabric using long fibers having a core-sheath structure, on both sides of the PTFE porous membrane in order to prevent damage and the occurrence of pin holes, since the membrane itself is thin (JP6-218899A). A filter medium using such a PTFE porous membrane has an excellent property, since its surface area is greater compared to that of a conventional filter medium.
However, the applicant found in his further study that a trace amount of total organic carbon (hereafter referred to as "TOC", wherein the TOC means a total amount of various gaseous organic substances such as, for example, dodecane, tridecane, butyl hydroxytoluene (BHT), phosphate ester, dioctyl phthalate, siloxane or the like) was generated in using the conventional filter medium proposed in JP5-202217A and JP6-218899A mentioned above, for example, as an air filter in a clean room. In manufacturing semiconductor, liquid crystal or the like, the product quality might be deteriorated by the presence of high TOC in a cleaned air space such as a clean room or the like. An air filter unit is composed of not only a filter medium but also members such as a frame housing for supporting the filter medium, a sealing compound that seals the gap between the frame and the filter medium, a gasket, a spacer, a separator and the like. Therefore, the generation of organic substances from these members should be also taken into account.
In order to solve this problem, it can be considered whether to construct an air filter unit composed of members from which little organic substances are generated. However, the generation of organic substances cannot be actually restrained efficiently merely through such a method.
Thus, an object of the present invention is to provide an air filter unit in which the generation of organic substances is restrained and a method for manufacturing the same.